1. Field of the Invention
This invention relates to a method and an apparatus using a baffled arc chamber, and more particularly use of a baffled arc chamber in an ionization source as part of an ion implanter for depositing ions on wafers.
2. Description of the Related Art
In the manufacturing of silicon wafers, ion implantation is an effective and accurate method to dope required regions through a selective addition of properly charged ions. In an ion implanter dopant atoms are ionized and isolated, accelerated, formed into an ion beam, and swept across a wafer. These dopant atoms bombard the silicon wafer, enter the surface and eventually come to rest below the surface of the wafer.
An ion implanter involves many sophisticated sub-systems, each sub-system performing a specific action on the ions. In ion implanters, gas and solid sources are used. The gas sources typically are contained in gas cylinders and are connected to the ion source sub-system through mass flow meters controlling the gas. When solids are used, the solids are in powder form and vaporized into the ion source sub-system.
The ion source sub-system consists of a gas inlet or inlets, control meters, and typically a separate inlet for vapors, whenever solid powders are used. The inlet or inlets lead into an ionization source chamber, sometimes also referred to as an arc chamber. An electrically activated filament typically is used in the chamber. When the filament is activated, electrons are formed. As gas or vapor is introduced into the chamber, the electrons collide with the dopant source molecules of the gas or vapor and positively charged ions are created. Gas molecules are transformed into positively charge ions that may or may not be used in the ion beam. In addition to the desired ions, unneeded atoms or ions are created. As example, when the gas BF3 is used in an ionization chamber, the following species (ions and atoms) are created in the chamber: B+, BF+, BF2+, BF3, F+, F2+, B11+, B11F+, B11F2+, and BF3+. In typical applications, only the B+ ion is desired. The other species created in the chamber are not wanted on the wafer.
Ion separation or selection is conducted by a mass analyzer. The mass analyzer creates a magnetic field in which the species enter. The species leave the chamber with voltages of 15 to 40 keV (thousand electron volts) which translates to a relatively high speed of travel through the magnetic field. In the magnetic field, each species is bent into an arc and travels along a specific radius. The specific radius of travel is determined by the individual species, the speed of the species, and the strength of the magnet field. At the end of the mass analyzer is an exit slit in which only one species can exit. It is expected that some contamination will exist when secondary collisions are made between species. It has been found that if the gas or vapors are efficiently disassociated into the species, the less likely will be these secondary collisions. The strength of the magnet therefore is adjusted to properly align the path of the ion with the exit slit.
Upon leaving the analyzer, the ions move into an acceleration tube. The acceleration tube brings the ions to a high enough velocity to gain the necessary momentum to implant the ions into the wafer. The acceleration tube has anodes along its sides; the negative charge of the anodes gradually increases as the positively charged ions accelerate along the tube. Adjusting the charge on the anodes determines the acceleration and the momentum of the ions, and ultimately how deep the dopant ions will be implanted into the wafer.
The stream of ions exiting the tube is often referred to as an ion beam, however, it is actually an electric current. The current level determines the number of ions implanted over a time interval. The higher the current, the more ions that are implanted per minute. Ion implanters are commonly categorized as to the beam current that they produce. Medium current implanters produce currents of about 0.5 to 1.7 milliamperes (mA), while high current implanters produce currents of about 10 mA. Regardless of the classification of the implanter, it is a key requirement that the implanter have a consistent beam current to effectively dope the wafer.
The ionization or arc chamber described above is usually used by medium current implanters. In a xe2x80x9cFreeman Ion Sourcexe2x80x9d the filament in the chamber extends across the chamber, electrons radiating from the filament. An alternate or more common application involves a coiled over filament that is attached at one end of the chamber opposite a repellar plate. Electrons are generated by the filament and repelled by the repellar plate. This setup is known as a xe2x80x9cBernas Ion Source.xe2x80x9d Since it is highly desired to have more electrons react (collide) with more dopant atoms, the electron distribution of the xe2x80x9cBernasxe2x80x9d setup has found to be more effectively. To further create greater reaction rates, some gas chambers have incorporated multiple gas inlets leading into the chamber. Additional gas inlets have required the need for additional flow meters and additional control requirements. Over time, due to waste by-products these gas inlet lines become constricted and must be cleaned out. The more inlet lines to the arc chamber, the more lines that must be cleaned out and maintained. Many ion sources also incorporate a separate inlet for vapors. From a maintenance perspective, inefficient reaction of dopant gas or vapors in the arc chamber requires more frequent cleaning.
Because of the need to maintain a consistent beam current, control systems are set in place to adjust filament voltage and gas inflow to assure that the proper amount of the required ions are generated. At times it may take up to 15 minutes to automatically xe2x80x9ctunexe2x80x9d a filament for the proper voltage in consideration of the rate of reactions that are taken place. If the reaction rates between electrons and gas or vapors can be made more efficient this xe2x80x9ctuningxe2x80x9d time can be reduced.
Creating more electrons may translate to more collisions forming more charged usable ions, however, there must be a greater number of such ions to collide with. This requires greater gas (or vapor) flow. Gas (vapor) flow, however, is limited by the gas (vapor) inlet (s), the size the chamber, and other operating considerations such as pressure within the chamber. Increasing gas (vapor) flow also may mean that there will be more gas or vapor that is not used in the process and is wasted. An improved gas chamber, which better distributes gases and vapors, is clearly desirable for ion sources used in ion implanters.
Accordingly, one object of the present invention is to provide an ionization or arc chamber that distributes the dopant source gas or vapor entering the arc chamber, so that there is a greater opportunity for atoms of the gas or vapor molecules to react (collide) with electrons generated by a filament in the chamber.
Another object is to provide a single entry into the arc chamber for gases and vapors, eliminating multiple source inlets.
Briefly described, the invention contemplates a gas or arc chamber that includes a plenum that is mounted at the base of the arc chamber. Gas enters the plenum through a single inlet and redistributed in the plenum. Vapors from solid powders enter through separate inlets, however, the vapors are also collected in the plenum. The plenum is connected to a baffle. The baffle is inserted into the base of the arc chamber. Gas or vapor enters the plenum and is directed into a sub-plenum of the baffle. The baffle distributes the gas or vapor throughout the arc chamber, maintaining a uniform volume distribution inflow to the arc chamber. The greater distribution of the gas or vapor in turn allows a greater chance of interaction or collision between dopant atoms of the gas or vapors and electrons formed by the filament resulting in a more uniform ion plasma.